This invention pertains to membrane separation processes and systems that provide for increased recovery of the sought permeant. For purposes herein, the permeant is the component for which the membrane is intended to selectively permeate. Thus the fluid feed and the permeate fraction would contain permeant and the retentate fraction may contain permeant. The retentant is the component of the feed which is intended to be selectively rejected by the membrane. A fluid feed may contain two or more components and thus there may be two or more retentant and two or more permeants. Except as otherwise stated, where more than one permeant exist, the permeant shall mean the most desired component in the permeate fraction, e.g., normal paraffins where the feed is a naphtha fraction. Similarly, where more than one retentant exist, the retentant shall mean the most desired component in the retentate fraction, e.g., branched paraffins where the feed is an effluent from an isomerization of a C5 and C6 feedstock.
Membranes have been proposed as an alternative separation unit operation. The membrane separations have had limited commercial success in displacing alternative separation unit operations such as distillation, selective sorption, liquefaction and crystallization. In some instances, the capital cost of membranes for a given recovery of permeant is a significant deterrent from the use of a membrane separator, particularly for large-scale commercial processes. For example, refineries process large volumes of hydrocarbon feeds including the difficult separations of closely boiling components such as isomers and aromatic and aliphatic compounds of similar molecular weights. Nevertheless, distillation and sorption separation processes are still the primary processes for these difficult separations.
One of the disadvantages to the use of membrane separation processes for refinery applications is that extremely large membrane surface areas would have to be provided in order to achieve the sought separations. Many membranes that have been proposed for refinery and chemical process uses have been made with relatively thick barrier layers so as to assure that the sought separation can be achieved. See, for instance, U.S. Pat. No. 5,069,794 disclosing microporous membranes containing crystalline molecular sieve material; and U.S. Pat. No. 6,090,289, disclosing a layered composite containing molecular sieve that could be used as a membrane. The selectivities of the membranes can be quite high. For example, U.S. Pat. No. 6,818,333 discloses thin zeolite membranes that are said to have a permeability of n-butane of at least 6·10−7 mol/m2·s·Pa and a selectivity of at least 250 of n-butane to isobutane. Recently, Bourney, et al., in WO 2005/049766 disclose a process for producing high octane gasoline using a membrane to remove, inter alia, n-pentane from an isomerized stream derived from the overhead of a deisohexanizer. In a computer simulation based upon the use of an MFI on alumina membrane, example 1 of the publication indicates that 5000 square meters of membrane surface area is required to remove 95 mass percent of n-pentane from the overhead from a deisohexanizer distillation column. At the flow rate of feed to the permeator (75000 kg/hr. having 20.6 mass percent n-pentane), the flux of n-pentane used in the simulation appears to be in the order of 0.01 gram moles/m2·s at 300° C.
Thus, the costs for commercially implementing such a membrane separation system render it not competitive with respect to an adsorption separation system even if the entire naphtha stream were treated to remove contaminants potentially deleterious to the sorbent.
Additionally, membrane separators have fixed surface areas and to provide a product of constant purity over a range of feed rates and feed compositions, changes in the driving force, e.g., partial pressure or concentration gradients, may need to be made. Similarly, if a membrane becomes fouled resulting in the loss of permeance, the recovery of permeant will decrease unless changes in driving force are made to compensate for such loss.
Alternatives to, for instance, distillation systems for separations are sought due at least in part to the high energy consumption of the distillation process. Selective sorption processes can be more energy efficient than distillations but often involve more capital expense than a distillation system. Membrane separation systems generally offer an energy efficient separation as the driving force for the permeation is typically a differential in partial pressure or concentration.
Accordingly, a need exists to develop membrane separation systems that are an economically attractive alternative to conventional separation systems, especially where large volume streams must be treated. Membrane separation systems are also desired that can provide constant product purity over a wide range of feed rates and compositions without the complexities of changing the driving force for the separation.